In football, the formation describes how the players in a team are positioned on the field. Many variations are possible on both sides of the ball, depending on the strategy being employed. On offense, the formation must include at least seven players on the line of scrimmage, including a centre to start the play by snapping the ball. There are no restrictions on the arrangement of defensive players, and, as such, the number of defensive players on the line of scrimmage varies by formation. Therefore, the deployment and tactics of defensive players are bound only by the imagination of the play designer and the line of scrimmage.